1. Field of the Invention
The present invention relates to a semiconductor device capable of preventing a soft error caused by a neutron.
2. Description of the Background Art
Recently, in meetings of academic societies, articles or the like, various reports have been made on a soft error caused by a neutron in a DRAM (Dynamic Random Access Memory) or an SRAM (Static Random Access Memory) formed on a silicon substrate, for example by Ziegler et al. of IBM (International Business Machines Corp.) (J. F. Ziegler et al., J. Appl. Phys., 52(6), 1981, p. 4305), Mckee et al. of TI (Texas Instruments Inc.) (W. R. Mckee et al., IRPS Proceedings, 1996, p. 1) and Tosaka et al. of Fujitsu (Y. Tosaka et al. IEEE Trans. Nuel. Sci., Vol. 44, 1997, p. 173).
According to the reports, neutrons in a region with high energy level, specifically neutrons of about several tens of MeV or more react with silicon atoms and generate electric charges by elastic scattering or burst reaction. Then, electrons of the electric charges are collected to a storage node portion, so that the electric charges stored in a memory cell are changed and cell data is destroyed. This is a difficult problem to be solved because a larger number of electric charges are generated by the reaction as compared with a conventional soft error caused by an .alpha.-particle.
Meanwhile, it has been predicted by Robert Baumann et al. of Japan TI (R. Baumann et al. IRPS Proceedings, 1995, p. 297) that the soft error can be caused by reaction of .sup.10 B included in a planarizing film before a metal interconnection of a device is formed and thermal neutrons (with energy level of about 0.05 eV) even in a region with neutrons of low energy level. A specific reaction formula is as follows. EQU .sup.10 B+n.fwdarw..alpha.(1.471 MeV)+Li(839 KeV)+.gamma.(479 KeV)
The .alpha.-particle or Li which is generated by the above mentioned reaction reacts with silicon to generate electric charges, whereby data is destroyed at a storage node. It is noted that FIG. 11 is a schematic diagram shown in conjunction with generation of the electric charges caused by the above mentioned two types of reactions. In FIG. 11, a BPSG (Boro-Phospho Silicate Glass) layer 17 is formed on a silicon substrate 16 as a planarizing film including .sup.10 B.
Although two types of the soft errors caused by neutrons have been reported as described above, the actual degrees of neutrons with different energy levels contributing to the soft error have been unknown. Then, the inventors of the present application manufactured two different types of SRAMs, one including .sup.10 B in a planarizing film and the other without .sup.10 B, and performed two types of experiments in the following.
In the first experiment, .sup.252 Cf (californium) is used as a source of a neutron, covered with paraffin and converted to a thermal neutron, and directed to the SRAM. At the time, when a soft error ratio of the device including .sup.10 B exceeds that of the device without .sup.10 B, it is apparent that the soft error is caused by reaction of the thermal neutron and .sup.10 B. As the result of the experiment showed a difference in soft error ratios of thirty times or more, it was found that the soft error is caused by the reaction of the thermal neutron and .sup.10 B.
Next, as a second experiment, the degree of the influence of by thermal neutron on an energy distribution of neutrons in the nature was investigated. More specifically, a neutron acceleration experiment in high altitude flight was performed and contribution of each reaction was evaluated. It has been reported that 100 times or more neutrons exist in high altitude than on the ground (T. Nakamura, 1987, "Altitude Variation of Cosmic-Ray Neutron", Health Phys. Vol. 53, p. 509.), so that evaluation can be made in a short period of time and while ignoring a soft error caused by a usual .alpha.-particle. The result of the experiment showed that about half of the soft errors caused by neutrons are those caused by thermal neutrons. Thus, the inventors of the present application came to an conclusion that in solving the problem of the soft error, the thermal neutron is as important as the fast neutron which has conventionally been said as cause for soft error.